I've played that game through a dozen times and I don't remember that.
It's very easy to miss because simply having them in your party and picking them won't do anything. You have to specifically go into FPP, something most people never do, which is why most people don't even know about it.
Here's a vid I found which can give you an idea (poor quality vid) https://www.youtube.com/watch?v=PQEPrtutUWE
To my knowledge the special sights don't have any real game purpose other than just being cool. Shame that most gamers never even discovered it. For tun, use the alignment sight on important NPCs or doors to restricted areas. You can actually see something (possibly surprising) about a certain NPC this way (no spoilers).
You can actually obtain this vision mode (which shows dark/light alignment, and can see characters through walls) from one of your companions - provided your influence with them is high enough, and you ask the right questions. It's one of the "hidden" force powers you can get. The difference is that for your character, you need to activate the power before you can use it, it works for a limited time only, and it works in 3P mode as well. So if you have that companion with you, it's probably still easier to switch to them and go to 1P mode.
Not sure which NPC you're referring to - I assume it's not the obvious big-reveal one... I'm playing this through again at the moment (finally managed to get the latest TSLRCM combined with a bunch of graphics-enhancing mods to actually work without crashing) so I'll keep my eye out...